The technology known as MIMO, Multiple Input, Multiple Input, is advantageous in wireless communications systems since MIMO offers the possibility of increased spectrum efficiency and increased peak rates as compared to non-MIMO technologies.
Implementations of MIMO in EGPRS/EGPRS2 will have to take into account the structure of the EGPRS/EGPRS2 packet data channels, which use burst transmissions, and where one radio block comprises four bursts. In so called BTTI mode (Basic Transmission Time Interval), the bursts are transmitted in four consecutive TDMS frames, i.e. over a total of 20 ms, while in so called RTTI mode (Reduced TTI), the four bursts are transmitted in two consecutive TDMA frames, i.e. over a total of 10 ms. A burst in EGPRS/EGPRS2 comprises a training sequence, as well as payload. The payload comprises a header, as well as the so called q-bits, which partially signal to the receiver which so called MCS, Modulation and Coding Scheme that has been used.
The use of the word “partially” is used here with reference to the information given by the q-bits regarding which MCS that has been used can be exemplified by the following: If, for example, in an EGPRS system, the q-bits are 0 0 0 0 0 0 0 0, this sequence of q-bits signals to the receiver that either MCS5 or MCS6 has been used. This means that even with the aid of the q-bits, the receiver still needs to determine the exact MCS that has been used, although the q-bits have aided in narrowing the number of alternatives. As mentioned, the payload comprises a header, and there are different types of headers. In our example, by knowing that either MSC5 or MCS6 is used, the receiver knows the header type (which is common to MCS5 and MCS6), so the receiver can now decode the header and determine the exact MCS.
The use of MIMO has been proposed in EGPRS and EGPRS2. In a straightforward implementation of MIMO in EGRS and EGPRS2, each of two (or more) data streams, also sometimes referred to as “layers”, is transmitted from its own antenna. In such an implementation, although advantageous in many aspects, the signal strengths of the data streams will vary over time, and the rates of (successful) detection will vary accordingly.
In conclusion, MIMO implementations of EGPRS/EGPRS2 need to be designed to maximize the link performance, while maintaining, as much as possible, backwards compatibility with the GSM interface.